US2935421A - Coating iron and its alloys with magnesium - Google Patents

Description

U i e l w Pamo COATING IRON AND rrs ALLOYS wrrn MAGNESIUM No Drawing. Application December 13, 1957 Serial No. 702,527 5 Claims. Cl. 117-52 The invention is. directed toward forming a film of magnesium on iron and its alloys. It is especially directed towardan improved method of providing protection to steel which is subjected to electrochemical action.

1 vOne of the most destructive types of corrosion of metal generally and of predominantly iron alloys particularly is electrochemical incharacter. Some few metals provide, their own protection because 'they form on their surfaces a non-porous and continuous protective film either by their being deposited thereon an insoluble deposit formed by the oxidation of 'an anodic material in contact therewith or by the interreaction of the surface of the metal to be protected and oxygen. Metals of the selfprotecting type are A1,,Ni, Si, Co, and Cr. Even these metals cannot protect themselves against electrochemical attacks under certain conditions, e.g., in the presence of chlorides.

-'Due to the high strength, abundance, and capacity to form a great number of alloys having specific properties, iron is the most widely employed metal today. Although iron alloys form a scale of Fe O on the surface which provides some protection against further attack-s when in dry air, suchprotection disappears in the presence of moisture. Therefore, with the possible exception of certain so-called stainless steels, iron alloys corrode at a high rate unless continuously protected against corrosive a'gencies'when subjected to electrolytic action. Particularly susceptable to corrosive attack are steel pieces which are in contact with brines, e.g., ocean water, and steel structural supports, pipes, cables, and plates, e.g., those of storage tanks; which are buried in the ground. An electrical potential often develops wherein portions of such steel pieces frequently become the anode of an electrolytic or galvanic system, the brine or the fluids of the soil being the electrolyte, and other metal pieces in contact with the steel piece and/or other parts of the same .ste'el piece being the cathode.

.'This potential is due to a number of causes among which are variations in the concentrations of oxygen and other; chemicals present in the brine or soil; areas of strain in the steel; variations of the steel alloy; variations in the soil conditions; stray currents from near-by direct electrical currents; and the character of different steel alloys and other dissimilar metals in contact withthe steel such as fittings, braces, cables, connecting lines,transfer lines, pumps, and signal systems.

- Electrolytic action in many instances defies accurate explanation and appears to be at times capricious in its behavior. 7 The presence of a coating of wet rust, for example, has been found to accelerate the corrosion whereas dry rust has inhibited the corrosion, a condition possibly due to the wet rust preventing further oxidation and formation of protective iron oxide scale.

j' The steel'is gradually dissolved so long as it continues to be theanode in such a galvanic action.

[To combat the corrosive agents on steel parts which are caused by the current-producing galvanic action in brine and in the soil wherein the steel anode is consumed, methods have been developed which comprise making the steel or areas of the steel cathodic. Such protection is usually referred to as cathodic protection. One way of accomplishing this is to place a more electropositive metal in contact with the steel. The more electropositive metal then becomes the anode, making the steel cathodic in relation thereto, and as a result thereof, the more electropositive metal is sacrificed and the steel is notcorroded away. Magnesium, aluminum, zinc, and cadmium are among the metals most commonly employed for this purpose.

Magnesium has .long been'kuown to be especially desirable for this purpose because it ranks above other common metals in the electromotive (sometimes called galvanic) series of metals and possesses suflicient strength and resistance to the chemical action of fluids in the soil to' be highly servicable for this purpose.

For best cathodic protection, the protecting metal e.g., magnesium, should be in firm direct contact with the steel to be protected and preferably cover it, e.g., form a continuous film or coating thereover. The employment of magnesium for this purpose, however, has been accompanied by the disadvantage of its not lending itself by known methods to a satisfactory cladding or coating process for steel.

There is, therefore, a need for an improved method of forming a continuous adhesive coating or film of mag nesium on steel and for magnesium-clad or magnesiumcoated steel products The principal object of the invention is to provide such method and products. Another object is to provide a method of protecting steel against corrosion whichis th result of electrochemical action.

Consequently, the invention is a novel magnesium-clad steel product and the method of cladding or coating" a surface of steel with magnesium which consists essentially of superimposing molten magnesium on a molten saline flux of greater density than the magnesium, passing gaseous TiCL; up through the molten flux and into the molten magnesium according to the method described in U.S. Patent 2,779,672, immersing a steel piece to be magnesium-coated into the molten magnesium in a position so that the surface to be coated is substantially transverse to the path of rising TiCL; gas which thereby causes the surfaceof the steel piece to 'be contacted with the TiCl gas and thereafter removing the steel-coated piece from the molten magnesium. After cooling the removed piece, it is recommended that any adhering flux then be waterrinsed from the steel piece and the piece dried.

Since, in cathodic protection the magnesium is sacrificed and hence slowly consumed, a heavier coating of magnesium on steel than that provided by the immersion of the steel in the molten magnesium may be desired. This is readily accomplished once a good bond has been provided between the steel and a layer or film of magnesium even though such film be thin. Additional protective magnesium may be provided on the steel according to a further embodiment of the invention which consists of casting magnesium about the magnesium-clad steel. No difiiculty is encountered in forming a firm bond between the adhering magnesium, already formed on the steel by immersion in the molten magnesium superimposed on the saline flux through which TiCl gas was passed, and one or more additional layers of magnesium formed about the initial film of magnesium. For example, by merely placing the steel thus-clad by immersion in a mold and pouring molten magnesium thereabout and cooling, an additional coating of any desired thickness can be obtained.

in practicing the invention, a suitable saline flux is placed in a melting pot. The pot may be aconventional one, e.g., one of steel supported in a conventional ceramic or brick furnace setting fired by gas. A TiCl inlet releases TiCl, at or near the bottom of. the pot. Best results are obtained by havinga generally horizontal perforated plate positioned near the bottom of the pot above the opening of the TiCl inlet to provide dispersionofthe TiCl as it rises through the contents of the pot. The flux is a mixture of fused halides. Alkali and alkaline earth chlorides including one or more fluorides are'usually employed. A mixture of the chloridesv of K, Mg. Ba, and sometimes Na, with some Calis most often employed. Examples in weight'percents of fluxes employed are:

The flux employed shouldnot be adverselyaffected by the TiCl gas, should not stratify-within itself appreciably, should have desirable wetting properties, and be heavier than the molten magnesium at the temperature employed. Any temperature above the melting point of magnesium and the flux and below the boiling point of magnesium and of the flux can be employed; The flux employed preferably has a lower melting pointthan that of magnesium. A temperature. of between about 660 and 800 C. is usually employed during the magnesium-coating operation.

The magnesium employed is preferably substantiallypure or contains up to about 0.15 percent Mn and itmay contain small amounts of Cu, Ni, Pb, Sn, Al, Si, Zn, and Ca. A trace of Fe may be tolerated-but it is highly desirable that the Fe content be as low as is'economically feasible. The magnesium of a purity similar to that employed in the example set out-hereinafter is generally used. 1

The ratio of fiux to magnesium is not highly criticalso long as the flux forms a continuous protective layer between the molten magnesium and the TiCl Without such protective flux layer the TiCl reacts immediately with the magnesium upon the entrance of theTiCl into the pot and prevents satisfactory. attainment of the objects of the invention.

The rate of admission of TiCl gas maybe controlled in the interest of economy so that there is sufiicient TiCl present to contact the steel surface to be coatedbut not great enough to. result in escape of any appreciable amount of TiCl from the surface of the molten magnesium.

The steel piece may be immersed or submerged by any known means, e.g., a hoisting means such as a block and tackle arrangement or hydraulic lift. The process lends itself to a continuous operation in which pieces of-steel are consecutively lowered, immersed, and after the magnesium coating has formed, be hoisted and carried away. The surface of the steel to be magnesium-coated is usually positioned substantially horizontally so that the face of surface to be coated will be contacted most elfec tively by the rising TiCl gas. The length of timeof immersion is not highly critical. the surface to be entirely contacted by the TiCl andthe molten magnesium is all that is required. Two minutes is usually ample and a time beyond five minutes is not generally recommended.

The example below illustrates a practice of the invention.

Example A lOO-gallon capacity steel pot in agas-fired brick furnace setting, having an inlet for TiCl at the bottom, was employed. 50 pounds offlux- A setout'hereinaboye were placed in the pot and melted, 50 pounds of mag- A time suflicient for nesium metal having the following specifications were placed in the pot:

Ca, and Fe and did not exceed 0.05 percent of any one of them. The balance was magnesium.

The temperature was raised to 700 6. The magnesium lay in a molten layer above the flux layer with a very thin film of adhering flux over the upper surface of the magnesium which served as a protective layer against oxidation by the air. TiCl gas was then admitted into the flux at the rate of 5 pounds per hour. The TiCl bubbled up through the flux and then upinto the magnesium, reacting therewith in a manner describedin U.S. Patent 2,779,672. Various sized pieces of mild' steel were then immersed into the magnesium layer, the

pieces being positioned while immersed so that the sur-.

0.001 inch thick and would not flake or scrape ofi bw being penetrated and scratched with a scalpel.

The magnesium-coated steel thus made may be: employed for any common use of steel upon which a metallic coating of magnesium is desirable. Among such uses are the manufacture of equipment of steel wherein the high strength of steel is desired but the steel isreactive with contacting, gases or liquids. The steel often contaminates the gases or liquids and is itself corroded there by. -It is possible to obtain the structural strength of steel but yet avoid the contamination of the contacting substances and short life of the steel by coating the-steel equipment with magnesium prior to its use. The chief use of steel according to the invention, however, is that of providing protection of steel against corrosion dueto electrolytic action which is set up when such steeLis placed in a medium, e.g., aqueous solutions of ionizable substance and in damp ground, in which electrolytic fluids are found and wherein the steel becomes the anode. Such a condition prevails in most naturally. occurring waters and soils and subsurface formations.

The formation of a continuous durable film withon without additonal layers of an anodic protective materialsuch as magnesium over the steel piece to be protected, prior to its being put into such electrolytic environment, can readily be seen to provide superior protection tothe steel piece. A thickness of magnesium of' any desired thickness can be cast around a steel piece whichis already. coated by an adherent magnesium film to provide longer lasting protection. A bond once formed between magnesium and steel in the form of a magnesium film then permits subsequent bonding of additional layersof-=mag-. nesium to be cast around the magnesium coating without difficulty.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

. -1. The method of coating a steel surface witlimagnesium metal consisting essentially of immersing the, steel having asurface to be coated in molten magnesium which is superimposed on a layer of molten saline flux selected fromthe class consisting of a halide of anialkali metal,.,an

alkalineearth metal, andv mixtures thereof,,paS$ing;TiCl

nesium while positioning said steel surface so immersed so that it is contacted by the rising TiCh gas.

'2. The method of providing a protective coating on steel against corrosion resulting from electrochemical action consisting essentially of melting a saline flux having a density greater than that of molten magnesium, placing magnesium metal in the flux selected from the class consisting of a halide of an alkali metal, an alkaline earth metal, and mixtures thereof, heating and maintaining the temperature thereof above the melting point of the flux and magnesium to eifect stratification of the molten flux and magnesium, passing TiCl upwardly through the molten flux and into the molten magnesium, immersing the steel to be coated in the molten magnesium so thatit is contacted by the said TiCl, gas, thereby to form a protective film of magnesium on the steel, and thereafter removing the thus-coated steel from the molten magnesium.

3. The method of claim 2, wherein the removed magnesium coated steel is water-rinsed and dried.

4. The method of claim 2, wherein the halide flux consists of a mixture of a major proportion by weight of KCl and a minor proportion by weight of chlorides selected from the class consisting of MgCl CaCl Bacl SrCl NaCl, and mixtures thereof and between 1 and 3 percent of the fluorides selected from the class consisting of fluorides of Li, Ca, and Mg.

5. The method of claim 2, wherein the magnesium metal contains by weight between .05 and .15 percent Mn and not over .02 percent Cu, .001 percent Ni and .01 percent Sn, not over .05 percent of each of Al, Si, Zn, Ca, and Fe, and not over a total of 0.20 percent of Al, Si,

Zn, Ca, and Fe.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. THE METHOD OF COATING A STEEL SURFACE WITH MAGNESIUM METAL CONSISTING ESSENTIALLY OF IMMERSING THE STEEL HAVING A SURFACE TO BE COATED IN MOLTEN MAGNESUIM WHICH IS SUPERIMPOSED ON A LAYER OF MOLTEN SALINE FLUX SELECTED FROM THE CLASS CONSISTING OF A HALIDE OF AN ALKALI METAL, AN ALKALINE EARTH METAL, AND MIXTURES THEREOF, PASSING TICL4 GAS UPWARDLY THROUGH SAID FLUX AND INTO SAID MOLTEN MAGNESIUM WHILE POSITIONING SAID STEEL SURFACE SO IMMERSED SO THAT IT IS CONTACTED BY THE RISING TICL4 GAS.